It is well known in the art that polyoxymethylenes can be prepared by solution-polymerizing or bulk-polymerizing trioxane at relatively high temperatures and in the presence of an ionic polymerization catalyst. It is also known that polyoxymethylenes can be prepared by irradiating trioxane in solid state with ionizing radiation. (See U.S. Pat. Nos. 3,242,063 and 3,305,464.) However, it is a fact that polyoxymethylenes produced by the known prior art methods exhibit poor thermal stability and have to be subjected to esterification or etherification at their terminal groups for purposes of stabilization since otherwise the polyoxymethylenes produced would not be useful for commercial purposes, to wit, for use as plastic materials. Several methods for such stabilization of the terminal groups have been proposed. Thus, for example, according to one procedure, the polyoxymethylene is reacted with 2 to 20 parts by weight, per part of polyoxymethylene, of an anhydride of a saturated, monobasic, carboxylic acid. The reaction is carried out at about 50.degree. C. or above to cause esterification of the terminal groups (see U.S. Pat. No. 2,964,500). It is also known to effect such stabilization by reacting the polyoxymethylene with an alkyl acetal or the like to etherify the terminal groups of the polyoxymethylene. (See British Pat. No. 868,365.)
There has also been proposed a method for preventing coloration of the polyoxymethylene according to which the esterification procedure is carried out in the presence of a urea derivative (see French Pat. No. 1,364,410). Further, it has been suggested to prevent a significant lowering of the molecular weight of the polyoxymethylene in the course of the esterification by adding carbodiimide to the reaction system (see British Pat. No. 864,403).
It is an accepted view in this art that esterification is deemed to be preferable to esterification for stabilization purposes. As a matter of fact, the etherified polymer exhibits superior alkali-resistance as compared to the esterified polymer. Moreover, even in the case when the etherified polymer decomposes, no acids are produced which would tend to accelerate further decomposition. However, as is known, an acidic catalyst is customarily employed for the etherification purposes. It is generally recognized that the use of such acid or acidic catalyst is undesirable becaue it has a tendency to cause decomposition of the polymer in the course of the etherification (see M. Sittig, "Hydrocarbon Processing" Vol. 41, No. 11, p. 151).
Still another procedure for the production of polyoxymethylenes containing at least partially stable terminal groups, resides in the polymerization of trioxane in the liquid phase in the presence of ionic catalysts and in the presence of chain transfer agents, such as noncyclic acetals. (See U.S. Pat. No. 3,346,663.)
However, no data as to the molecular weight of the resulting polymers are disclosed or available. From an article by Werner Kern, a co-invertor of this procedure, published in Makromol. Chem., 83, 63 (1965), it can be reasonably concluded that this last mentioned prior art procedure is incapable of producing polyoxymethylenes of sufficiently high molecular weights so as to make them industrially interesting. The article in question, and particularly pages 63 through 65 thereof, contains graphs which indicate the relationship between the degree of polymerization and the ratio of [dimethylformal] / [trioxane] of polymers obtained by the solution polymerization of trioxane in a nitrobenzene solution. When 1 mole % of diemthy formal (formaldehyde dimethyl acetal) is added to trioxane, the resulting degree of polymerization will be 50, or 5,000 in terms of molecular weight, which is too low for commercial purposes. According to this article, the molecular weight of the polymer is low due to the water content. (See p. 64.) Yet, the molecular weight of the polymer in the presence of water and in the absence of dimethyl formal is 50,000. (See p. 64.) This means that the molecular weight is lowered to one-tenth by the addition of 0.76 wt. % of dimethyl formal only.
It has also been proposed to prepare polyoxymethylene exhibiting improved thermal stability by copolymerizing trioxane with olefin oxides (see U.S. Pat. No. 3,027,352). It has, however, been established, that the copolymers thus obtained are inferior in respect of their mechanical strength, as compared to homopolymers of trioxane.